Oxygen transport membranes function by transporting oxygen ions through a material that is capable of conducting oxygen ions and electrons at elevated temperatures. Such materials can be mixed conducting in that they conduct both oxygen ions and electrons or a mixture of materials that include an ionic conductor capable of primarily conducting oxygen ions and an electronic conductor with the primary function of transporting the electrons. Typical mixed conductors are formed from doped perovskite materials. In case of a mixture of materials, the ionic conductor can be yttrium or scandium stabilized zirconia and the electronic conductor can be a perovskite that will transport electrons, a metal or metal alloy or a mixture of the perovskite, the metal or metal alloy.
When a partial pressure difference of oxygen is applied on opposite sides of such a membrane, oxygen ions will ionize on one surface of the membrane and emerge on the opposite side of the membrane and recombine into elemental oxygen. The free electrons resulting from the combination will be transported back through the membrane to ionize the oxygen. The partial pressure difference can be produced by providing the oxygen containing feed to the membrane at a positive pressure or by supplying a combustible substance to the side of the membrane opposing the oxygen containing feed or a combination of the two methods.
Typically, oxygen transport membranes are composite structures that include a dense layer composed of the mixed conductor or the two phases of materials and one or more porous supporting layers. Since the resistance to oxygen ion transport is dependent on the thickness of the membrane, the dense layer is made as thin as possible and therefore must be supported. Another limiting factor to the performance of an oxygen transport membrane concerns the supporting layers that, although can be active, that is oxygen ion or electron conducting, the layers themselves can consist of a network of interconnected pores that can limit diffusion of the oxygen or fuel or other substance through the membrane to react with the oxygen. Therefore, such support layers are typically fabricated with a graded porosity in which the pore size decreases in a direction taken towards the dense layer or are made highly porous throughout. The high porosity, however, tends to weaken such a structure.
U.S. Pat. No. 7,229,537 attempts to solve such problems by providing a support with cylindrical or conical pores that are not connected and an intermediate porous layer located between the dense layer and the support that distributes the oxygen to the pores within the support. Porous supports can also be made by freeze casting techniques, as described in 10, No. 3, Advanced Engineering Materials, “Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues” (2008) by Deville, pp. 155-169. In freeze casting, a liquid suspension is frozen. The frozen liquid phase is then sublimated from a solid to a vapor under reduced pressure. The resulting structure is sintered to consolidate and densify the structure. This leads to a porous structure having pores extending in one direction and that have a low toruosity. Such supports have been used to form electrode layers in solid oxide fuel cells. In addition to the porous support layers, a porous surface exchange layer can be located on the opposite side of the dense layer to enhance reduction of the oxygen into oxygen ions. Such a composite membrane is illustrated in U.S. Pat. No. 7,556,626 that utilizes two phase materials for the dense layer, the porous surface exchange layer and the intermediate porous layer. These layers are supported on a porous support that can be formed of zirconia.
As mentioned above, the oxygen partial pressure difference can be created by combusting a fuel or other combustible substance with the separated oxygen. The resulting heat will heat the oxygen transport membrane up to operational temperature and excess heat can be used for other purposes, for example, heating a fluid, for example, raising steam in a boiler or in the combustible substance itself. While perovskite materials will exhibit a high oxygen flux, such materials tend to be very fragile under operational conditions such as in the heating of a fluid. This is because the perovskite will have a variable stoichiometry with respect to oxygen. In air it will have one value and in the presence of a fuel that is undergoing combustion it will have another value. The end result is that at the fuel side, the material will tend to expand relative to the air side and a dense layer will therefore, tend to fracture. In order to overcome this problem, a mixture of materials can be used in which an ionic conductor is provided to conduct the oxygen ions and an electronic conductor is used to conduct the electrons. Where the ionic conductor is a fluorite, this chemical expansion is restrained, and therefore the membrane will be less susceptible to structural failure. However, the problem with the use of a fluorite, such as a stabilized zirconia, is that such a material has a lower oxygen ion conductivity. As a result, far more oxygen transport membrane elements are required for such a dual phase type of membrane as compared with one that is formed from a single phase perovskite.
As will be discussed, the present invention provides a robust oxygen transport membrane that utilizes a fluorite as an ionic conductor and that incorporates a deposit of a catalyst in an intermediate porous layer located between a dense layer and a porous support to promote oxidation of the combustible substance and thereby increase the oxygen flux that would otherwise have been obtained with the use of a fluorite as an ionic conductor.